Method for separating boron isotopes

ABSTRACT

A method of separating boron isotopes  10  B and  11  B by laser-induced selective excitation and photodissociation of BCl 3  molecules containing a particular boron isotope. The photodissociation products react with an appropriate chemical scavenger and the reaction products may readily be separated from undissociated BCl 3 , thus effecting the desired separation of the boron isotopes.

BACKGROUND OF THE INVENTION

The invention described herein was made in the course of, or under, acontract with the U. S. ATOMIC ENERGY COMMISSION.

It relates to a method for separating boron isotopes and moreparticularly to a method based on laser-induced selective excitation andphotodissociation of BCl₃ molecules containing a particular boronisotope.

Boron highly enriched in ¹⁰ B has substantial utility as a neutronicpoison in nuclear reactors. The art discloses two methods for separationof ¹⁰ B and ¹¹ B. The first consists of the fractional distillation ofthe dimethyl- or diethyl-ether complex of BF₃. The second involves thelow temperature fractional distillation of BF₃ itself. Heretofore therehas been no known technique for separating these isotopes by selectiveexcitation and photodissociation of a boron-containing compound.

SUMMARY OF THE INVENTION

The isotopes ¹¹ B and ¹⁰ B may readily be separated by irradiatinggaseous BCl₃ containing both isotopes, selectively exciting those BCl₃molecules containing the desired B isotope, photodissociating theexcited BCl₃, reacting the photodissociation products with anappropriate chemical scavenger, and separating the undissociated BCl₃from the reaction products.

The necessary selective excitation is accomplished by irradiating agaseous mixture of the BCl₃ and the scavenger with light from the P or Rbranch of a CO₂ laser. Simultaneous irradiation of the mixture withultraviolet light at 213 to 215 nm then results in photodissociation ofselectively excited BCl₃ and prompts a reaction of the photodissociationproducts with the scavenger.

Preferably the CO₂ laser radiation is from the P branch whichselectively or at least preferentially excites those BCl₃ moleculescontaining ¹¹ B. The chemical scavenger may be any material that isgaseous, does not substantially absorb either the infrared or theultraviolet radiation, and is reactive with the photodissociationproducts of BCl₃ but substantially non-reactive with BCl₃ itself. Oxygenand various alkenes are suitable for this purpose. The requisiteultraviolet radiation is readily obtained from the output of Xe or D₂flashlamps filtered through unexcited BCl₃.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the infrared absorption spectrum of BCl₃ containing a naturalabundance of boron isotopes.

FIG. 2 displays ultraviolet absorption spectra of BCl₃ containing anatural abundance of boron isotopes.

FIG. 3 shows calculated ultraviolet absorption spectra of ground stateand excited BCl₃.

FIG. 4 shows a reaction vessel and ultraviolet filter used in an actualreduction to practice of the invention.

FIG. 5 shows the placement of the ultraviolet source and the reactionvessel within the flashlamp cavity.

FIG. 6 is a plot of measured enrichment versus pressure of BCl₃.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The isotopic abundance of natural boron is 19.8 at % ¹⁰ B with theremainder being ¹¹ B. The compound BCl₃ has a boiling point at 1 atm of12.5° C and absorbs the radiation of a CO₂ laser. This absorption takesplace in the ν₃ mode which for ¹⁰ BCl₃ has a frequency ν₃ = 995 cm⁻¹ andfor ¹¹ BCl₃ a frequency ν₃ = 956 cm⁻¹. The infrared absorption spectrumof BCl₃ at 0.5 torr and containing a natural ratio of boron isotopes isshown in FIG. 1. The isotope shift of 39 cm⁻¹ is a very large one and islocated within the manifold of rotational lines available from the CO₂laser. The CO₂ P branch lines largely coincide with the ¹¹ BCl₃absorption peak, whereas the R branch lines are matched to a largedegree by the ¹⁰ BCl₃ absorption peak. As shown in FIG. 1, the CO₂ P(20)line which is one of the strongest P branch lines is well within the ¹¹BCl₃ absorption peak.

Boron trichloride dissociates in the vicinity of 3.8 × 10⁴ cm⁻¹. Theproducts of dissociation are not accurately known but are quite probablythe free radicals Cl and BCl₂. As free radicals, both are highlyreactive and readily consumed by an appropriate chemical scavenger. Inaccordance with the preferred embodiment, BCl₃ molecules containing aparticular boron isotope are excited with the appropriate line of a CO₂laser, the excited molecules are preferentially dissociated byultraviolet radiation, and the dissociation products containing theparticular isotope react with a chemical scavenger.

The ultraviolet absorption spectra of natural abundance BCl₃ atpressures of 4 and 2.1 torr are given in FIG. 2. The absorption iscentered at 207.6 nm and free of any visible structure. It has a peakattenuation coefficient of α = 0.014 cm⁻¹ torr⁻¹. FIG. 3 displays the207.6 nm ultraviolet absorption fitted by a 5.0 nm FWHM Gaussian. Thisabsorption leads to dissociation of the BCl₃. These conditions areappropriate to a BCl₃ pressure of about 2 torr. Curve (a) depicts therelative magnitude and location of absorption by 50% of the naturallyavailable ¹⁰ BCl₃ molecules which have been promoted to the ν₃ mode byan infrared pulse of appropriate wavelength, while curve (b) displaysthe same information for excited ¹¹ BCl₃. Since the strongest lines ofthe CO₂ laser are absorbed by ¹¹ BCl₃, it is preferable topreferentially dissociate the ¹¹ BCl₃ and scavenge the dissociationproducts, leaving the gas enriched in ¹⁰ BCl₃. The ¹⁰ B can then bereadily recovered by conventional reduction of BCl₃ with H₂.Alternatively, if desired, the ¹⁰ BCl₃ can be preferentially dissociatedin accordance with this method.

The ultraviolet wavelength necessary to dissociate excited BCl₃ isdependent on the wavelength of the infrared radiation used to producethe excitation. It is in the spectral region of 213 to 215 nm. There arepresently no lasers available having outputs in this wavelength;however, an intense continuum source such as Xe or D₂ lamps can readilybe used if first filtered by being passed through a quartz cellcontaining BCl₃. The quartz passes little radiation below ˜ 190.0 nm andthe BCl₃, being unexcited and in the ground state, strongly attenuatesthe radiation centered around 207.6 nm indicated by the solid curve ofFIG. 3. As a consequence, radiation filtered by this cell can onlydissociate excited BCl₃. Although the filtering action will result indissociation of BCl₃ in the filter cell, recombination will ratherquickly occur, so that the filter cell will exhibit little if any aging.

A critical feature of the method of this invention is the presence of anappropriate chemical scavenger to react with the dissociation productsof the selectively excited BCl₃. An appropriate scavenger should begaseous, absorb little or none of the ultraviolet or infrared radiation,and react readily with the dissociation products of BCl₃, but not theBCl₃ itself. Suitable scavengers include O₂ and various alkenes.

Using the apparatus shown in FIGS. 4 and 5, the method of the inventionwas actually reduced to practice in the following manner. Reactionvessel 1 3 mm in diameter and having Brewster angle windows 2, 2'contained mixtures of 1 to 6 torr BCl₃ and 20 to 25 torr O₂. Surroundingreaction vessel 1 is filter vessel 3 25 cm long and 24 mm in diameterwhich contained BCl₃ at 600 torr. Reaction vessel 1 and filter vessel 3were placed at one focus of elliptical flashlamp cavity 4, while a Xeflashlamp 5 was placed at the other focus. Flashlamp 5 discharged 750joules of electrical energy in 250 μs. The ultraviolet filter 8 provideda 12 to 1 contrast ratio between 207.6 nm and 215.0 nm. Infraredradiation 6 at 4 × 10⁴ W/cm² was provided from a CO₂ laser (not shown)operating on the P(20) line.

The infrared radiation 6 was propagated axially through reaction vessel1 containing a desired (BCl₃,O₂) mixture at the peak of the ultravioletpulse. Five pulses of combined ultraviolet and infrared radiationdissociated about half of the initial amount of BCl₃ present in reactionvessel 1. Infrared radiation 6 alone would not dissociate any BCl₃,whereas the filtered ultraviolet radiation required about 15 pulses toinduce the same reaction as a single combined ultraviolet and infraredpulse. The product of the reaction between the scavenger O₂ and thedissociation products is thought to be (BOCl)₃.

The results of mass spectrometer analysis of the residual BCl₃ inirradiated mixtures of 1 to 6 torr of BCl₃ in 20 to 25 torr O₂ are shownin FIG. 6. The enrichment factors indicated in FIG. 6 can be increasedby increasing the power level of the ultraviolet source.

What I claim is:
 1. A method of separating isotopes of boron whichcomprises(a) obtaining a gaseous mixture of BCl₃ containing both boronisotopes and a scavenger, said scavenger being substantially nonreactivewith BCl₃ but highly reactive with photodissociation products of BCl₃,(b) irradiating said mixture with radiation from the P or R branch linesof a CO₂ laser to preferentially excite those BCl₃ molecules containinga particular boron isotope, (c) simultaneously irradiating said mixturewith ultraviolet radiation at a wavelength which will photodissociatethose BCl₃ molecules excited by said CO₂ laser radiation but not thoseBCl₃ molecules not excited by said CO₂ laser radiation, and (d)separating undissociated BCl₃ from the reaction products of saidphotodissociation and said scavenger.
 2. The method of claim 1 whereinsaid ultraviolet radiation has a wavelength in the range of 213 to 215nm.
 3. The method of claim 2 wherein said ultraviolet radiation isfiltered by passage through unexcited BCl₃ before irradiating saidmixture.
 4. The method of claim 1 wherein said BCl₃ in said mixture isat a pressure of about 2 torr.
 5. The method of claim 1 wherein said CO₂laser radiation is from the P branch of said CO₂ laser.
 6. The method ofclaim 5 wherein said CO₂ laser radiation is from the P(20) branch ofsaid CO₂ laser.
 7. The method of claim 1 wherein said scavenger is O₂.8. A method of separating isotopes of boron which comprises(a) obtaininga gaseous mixture of BCl₃ containing both boron isotopes and O₂, saidmixture comprising 1 to 6 torr of BCl₃ and 20 to 25 torr of O₂, (b)irradiating said mixture with 10.6 μm radiation from the P branch of aCO₂ laser to preferentially excite those molecules of BCl₃ containing ¹¹B, (c) simultaneously irradiating said mixture with ultravioletradiation at a wavelength between 213 and 215 nm to photodissociateexcited molecules of BCl₃, and (d) separating undissociated BCl₃ fromthe reaction products of said photodissociation and said O₂.
 9. Themethod of claim 8 wherein said ultraviolet radiation is filtered throughunexcited BCl₃ before irradiating said mixture.